This work is concerned with the isolation and characterization of proteoglycans obtained from an established line of rabbit endothelial cells. Heparan sulfate proteoglycans have been isolated by a procedure which entails gel filtration, precipitation with concanavalin A, and polyacrylamide gel electrophoresis. A monospecific anti-serum directed against heparan sulfate proteoglycans has been raised in the goat and its properties are now under study.